feat: add mobile responsiveness, design consistency, and micro-interactions

- Add mobile hamburger menu with slide-out nav drawer (AppLayout)
- Make modals responsive: full-width on mobile, slide-up animation
- Scratchpad becomes full-screen overlay on mobile with backdrop
- Folder sidebar hidden on mobile, opens as slide-over drawer
- Tree editor shows "Desktop Required" gate on mobile
- Stack action buttons vertically on mobile (sessions, detail pages)
- Increase touch targets throughout (buttons, close icons)
- Add CSS animations: fade-in, slide-in-left, scale-in, btn-press
- Add card hover lift effect and consistent border highlights
- Standardize page padding (px-4 py-6 sm:px-6 sm:py-8)
- Responsive headings (text-2xl sm:text-3xl)
- CustomStepModal goes full-screen on mobile
- Tighten auth page spacing on mobile

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/index.css

@@ -84,8 +84,59 @@
   }
 }
 
+/* Animations */
+@keyframes fade-in {
+  from { opacity: 0; }
+  to { opacity: 1; }
+}
+
+@keyframes fade-in-up {
+  from { opacity: 0; transform: translateY(4px); }
+  to { opacity: 1; transform: translateY(0); }
+}
+
+@keyframes slide-in-from-left {
+  from { transform: translateX(-100%); }
+  to { transform: translateX(0); }
+}
+
+@keyframes slide-in-from-bottom {
+  from { opacity: 0; transform: translateY(16px); }
+  to { opacity: 1; transform: translateY(0); }
+}
+
+@keyframes scale-in {
+  from { opacity: 0; transform: scale(0.95); }
+  to { opacity: 1; transform: scale(1); }
+}
+
 @layer utilities {
   .text-gradient-brand {
     @apply bg-gradient-brand bg-clip-text text-transparent;
   }
+
+  .animate-fade-in {
+    animation: fade-in 200ms ease-out;
+  }
+
+  .animate-fade-in-up {
+    animation: fade-in-up 200ms ease-out;
+  }
+
+  .animate-slide-in-left {
+    animation: slide-in-from-left 200ms ease-out;
+  }
+
+  .animate-slide-in-bottom {
+    animation: slide-in-from-bottom 200ms ease-out;
+  }
+
+  .animate-scale-in {
+    animation: scale-in 150ms ease-out;
+  }
+
+  /* Button press feedback for primary action buttons */
+  .btn-press {
+    @apply active:scale-[0.98] transition-transform;
+  }
 }